Biodiesel is generally prepared by the transesterification of natural oil or fat of vegetable or animal origin. The transesterification is carried out by acid or base catalysis. As the rate of reaction of the base catalyst is much faster than the acid catalyst, base catalysts are used, most often, commercially. For an alkali catalyzed transesterification, the glycerides and alcohol must be free from water and free fatty acids. Some of the natural oils or fats contain considerable amounts of free fatty acids, which interfere during the transesterification process. These free fatty acids are to be converted into their corresponding esters before transesterification. Thus, esterification forms an essential step in the production of biodiesel in case of oils containing free fatty acids.
Esterification is a very widely employed reaction in the organic process industry. Esters fall under a wide category ranging from aliphatic to aromatic with various multifunctional groups. Organic esters are most frequently used as plasticizers, solvents, perfumery and flavor chemicals and also as precursors to a gamut of pharmaceuticals, agrochemicals and other fine chemicals. Esterification reactions are conventionally carried out homogeneously using mineral acids such as sulfuric acid or Lewis acids as catalysts. These acids are corrosive and the excess acid has to be neutralized after the reaction, leaving considerable amount of salts to be disposed off into the environment.
Process for the preparation of biodiesel from low FFA and high quality oils may not be so difficult. The main challenge of the biodiesel technology is handling the multi feed stocks with high FFA. In case of high FFA oils, initially the fatty acid has to be esterified using acid catalyst like sulfuric acid followed by neutralization of the catalyst before going for alkali-based transesterification. The major problem of acid catalyst is the formation of salts during neutralization and also conversions are not very high like transesterification. This process involves high consumption of energy and the separation of the catalysts from the homogenous reaction mixtures is expensive and chemically wasteful.
Over 15 million tons of sulfuric acid is annually consumed as “unrecyclable catalyst, which requires expensive and inefficient separation of the catalyst from homogenous reaction mixtures for the production of industrially important chemicals, thus resulting in a huge waste of energy and large amounts of waste products. The green approach to chemical processes has stimulated the use of recyclable strong heterogeneous solid acids as replacement for such non-recyclable liquid acid catalysts. Tightening legislation on the emission of hazardous pollutants is driving the industry toward the implementation of innovative clean technology including the use of alternative heterogeneously catalyzed processes. The use of heterogeneous catalysts for these reactions offer several intrinsic advantages over their homogeneous counterparts like insolubility in the product i.e. ease of product separation, catalyst reuse and process advantages through reactor operation in continuous flow versus batch configuration. However, to maintain economic viability, a suitable heterogeneous catalyst not only minimizes the production of waste, but also exhibit activities and selectivity comparable or superior to the existing homogeneous catalysts.
Solid acid catalysts are of increasing importance for esterification in the production of bulk and fine chemicals. The use of solid catalysts offers an alternative to mineral acids and has received a lot of attention in the past years. Though, considerable amount of literature exists on esterification of simple aliphatic and aromatic acids using various solid acid catalysts like the resins, zeolites, heteropoly acids like tungstophosphoric acid and its amine salts and superacids like sulfated zirconia and niobium acid, only a few reports can be seen on the esterification of fatty acids. These catalysts have low densities of effective acid sites and thus cannot achieve adequate performance in acid-catalyzed reactions in the presence of water as replacement for homogeneous Bronsted acids in esterification [B. Y. Giri etal., Cat. Commu. 6, p. 788 (2005); S. Inagaki et al., Nature, 416, p. 304 (2002); K. Wilson et al., Applied Cat. A Gen. 228, p. 27 (2002); E. Cano-Serrano et al., Chem. Commu. 247, p. 246 (2003)]. These heterogeneous catalysts also have some drawbacks such as tedious preparation, leaching of the catalysts in to the reaction medium, change of active sites in the structure of the catalyst after reaction and hence not reusable and the starting materials are also expensive.
Michikazu Hara et al., [Angew Chem. Int. Ed., 43, p. 2955-2958 (2004)] reported the preparation of a solid acid catalyst by sulfonating naphthalene after carbonization at 200 to 250 deg C. However, it is a soft material and its aromatic molecules are leached out during liquid-phase reactions above 100 deg. C or when higher fatty acids are used as surfactants, so its catalytic activity is rapidly lost. The preparation of the reported solid catalyst was very tedious like heating the organic material with sulfuric acid at very high temperatures of 523 K under a flow of nitrogen for 15 hr in excess amounts of sulphuric acid (1:20 wt/vol). Excess sulfuric acid was removed from the product by vacuum distillation at the same temperature for 5 hr, which resulted in a black solid. The same authors have filed a European patent (EP 1,667,167, 2006) wherein the preparation of sulfonated amorphous carbon catalyst was reported from aromatic hydrocarbons such as benzene, naphthalene, anthracene, peryrene and coronene. However, a molar ratio of 1:6-36 of organic compound to sulfuric acid was used. In a separate US patent application (US 20060276668) the same authors disclosed the preparation of sulfonated composite solid acid catalyst containing the amorphous carbon, prepared by the above method from polycyclic aromatic hydrocarbons obtained by condensing two or more aromatic rings or tar, pitch, fuel oil or asphalt and incorporated a solid carbon component like carbon black, acetylene black, activated carbon, carbon nano tube or fullerene at high temperatures up to 450.degree. C. In this patent large amounts of sulfuric acid was also used, which necessitated removal of excess sulfuric acid by vacuum distillation. Masakakazu Toda et al., in their communication [Nature, 438, p 178 (2005); Catalysis Today, 116, p 157 (2006)] reported the preparation of another solid acid catalyst by sulfonating incompletely carbonized natural organic materials such as sugar, starch or cellulose. The preparation of this catalyst is also very tedious, involving heating of D-glucose or sucrose powder at 400.degree. C. under N.sub.2 flow for 15 hr to get the brown black solid which was further sulfonated by heating with more amount of concentrated sulfuric acid or fuming sulfuric acid (1:20 wt/vol) at 150.degree. C. under N.sub.2 for 15 h followed by hot water wash, which resulted in a black solid acid catalyst.